For example, as a thin-film transistor applicable to display devices such as liquid crystal displays, various control circuits and the like, a type which uses polycrystalline silicon (polysilicon) for the semiconductor layer is developed.
Polycrystalline silicon contains a plurality of crystals different from each other in size and orientation surface. Polycrystalline silicon is obtained by, for example, the following technique in which a laser beam is irradiated onto amorphous silicon and then the fused silicon re-solidifies into poly-crystals. But, during this operation, crystals may collide with each other to create a projection in a crystal boundary (to be referred to as grain boundary hereinafter).
If such polycrystalline silicon is used for the semiconductor layer of a thin-film transistor, a gate insulating film is brought into contact with the projection created in the grain boundary of the polycrystalline silicon. Therefore, in the boundary between the semiconductor layer and the gate insulating film, an interface state is easily created. If an interface state is produced in the boundary between the semiconductor layer and the gate insulating film, the drain current flows at a voltage lower or equal to the threshold voltage, thereby creating the tendency of degrading the sub-threshold characteristics. Further, there are further tendencies of increasing the leakage current flowing into the semiconductor layer from the gate electrode, or decreasing the voltage resistance of the gate insulating film, which decreases the reliability of the thin-film transistor.